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Gravity As A Lens

Gravity as a lens... General Relativity predicts that all things that carry energy must gravitate, including light! Much like the curving trajectory of a tennis ball in flight, a light-ray also bends because of the pull of gravity. This effect is undetectable when gravity is too weak, like near the surface of the Earth. But when light travelling towards us from stars and galaxies far away encounters the pull of entire galaxies during its trek, it can bend significantely. And we can see this effect dramatically in distorted images of the universe behind the curtain of massive galaxies that interrupt the light. Through such observations, we can infer how much energy and mass there is between us and some far away stars whose images are getting distorted. The conclusion is rather shocking: 26% of the stuff in the universe is some unknown substance that we cannot see! We call it Dark Matter, and we know it's there from the way it bends light that travels through it. Only 4% of the stuff in the universe is luminuous and can be seen! The remaining 70% is something even more mysterious; we call it Dark Energy. Gravitational lensing - the phenomenon where gravity's pull bends light - has become a crucial tool in measuring the stuff the universe is made of; and hence is helping us understand the cosmological history of our universe. It was also the phenomenon originally used (in the early 1900's) to confirm General Relativity as the correct description of gravity - at the expense of the centuries-old Newtonian gravity. Check out the accompanying video for a brief overview of this tentalizing twist that Nature has apparently provided us; as if trying to help us see what we otherwise cannot...

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